Deformation-related microstructures in magmatic zircon and implications for diffusion |
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| Authors: | Steven Michael Reddy Nicholas E Timms Patrick Joseph Hamilton Helen R Smyth |
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| Institution: | (1) Department of Applied Geology, The Institute for Geoscience Research, Curtin University of Technology, GPO Box U1987, Perth, WA, 6845, Australia;(2) Department of Earth Sciences, CASP, University of Cambridge, Cambridge, CB3 0DH, UK;(3) Present address: Intellection Pty. Ltd, 27 Mayneview Street, Milton, QL, 4064, Australia |
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| Abstract: | An undeformed glomeroporphyritic andesite from the Sunda Arc of Java, Indonesia, contains zoned plagioclase and amphibole
glomerocrysts in a fine-grained groundmass and records a complex history of adcumulate formation and subsequent magmatic disaggregation.
A suite of xenocrystic zircon records Proterozoic and Archaean dates whilst a discrete population of zoned, euhedral, igneous
zircon yields a SHRIMP U-Pb crystallisation age of 9.3 ± 0.2 Ma. Quantitative microstructural analysis of zircon by electron
backscatter diffraction (EBSD) shows no deformation in the inherited xenocrysts, but intragrain orientation variations of
up to 30° in 80% of the young zircon population. These variations are typically accommodated by both progressive crystallographic
bending and discrete low angle boundaries that overprint compositional growth zoning. Dispersion of crystallographic orientations
are dominantly by rotation about an axis parallel to the zircon c-axis 001], which is coincident with the dominant orientation of misorientation axes of adjacent analysis points in EBSD
maps. Less common <100> misorientation axes account for minor components of crystallographic dispersion. These observations
are consistent with zircon deformation by dislocation creep and the formation of tilt and twist boundaries associated with
the operation of <001>{100} and <100>{010} slip systems. The restriction of deformation microstructures to large glomerocrysts
and the young magmatic zircon population, and the absence of deformation within the host igneous rock and inherited zircon
grains, indicate that zircon deformation took place within a low-melt fraction (<5% melt), mid-lower crustal cumulate prior
to fragmentation during magmatic disaggregation and entrainment of xenocrystic zircons during magmatic decompression. Tectonic
stresses within the compressional Sunda Arc at the time of magmatism are considered to be the probable driver for low-strain
deformation of the cumulate in the late stages of initial crystallisation. These results provide the first evidence of crystal
plastic dislocation creep in zircon associated with magmatic crystallisation and indicate that the development of crystal-plastic
microstructures in zircon is not restricted to high-strain rocks. Such microstructures have previously been shown to enhance
bulk diffusion of trace elements (U, Th and REE) in zircon. The development of deformation microstructures, and therefore
multiple diffusion pathways in zircon in the magmatic environment, has significant implications for the interpretation of
geochemical data from igneous zircon and the trace element budgets of melts due to the potential enhancement of bulk diffusion
and dissolution rates. |
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| Keywords: | Arc magmatism Cumulate Electron backscatter diffraction (EBSD) Cathodoluminescence Crystal plasticity Dislocation creep |
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